Ontogeny: How do colonies form?

Castes and Roles * Disperser Morphs and New Colony Formation

Most commonly when we look at the development of social behaviors, we take an evolutionary perspective. It can also be interesting to look at it on a smaller scale, seeing how it pertains to the individual animal within its lifetime. This page looks at how castes and colonies develop, one mole-rat at a time.

Castes and Roles (top)

The animals in a colony are organized into several different morphs. The queen is large and fat, with exaggerated sexual characteristics: protruding nipples and a perforate vagina. She mates with one to three breeding males, who are also larger and fatter than the rest of the colony. The rest of the animals, the 'subordinates', can either be classified as disperser or non-disperser morphs. Disperser morphs are relatively larger than non-dispersers, and they also are characterized by laziness, doing less work than their brethren. The queen tends to mate with males from this caste. Non-dispersers, or 'workers', are smaller and thinner, and do most of the work in the colony, including digging tunnels, foraging for food, and caring for the young. (5).

Since the smaller and larger naked mole-rats have different roles in the colony, there is some question of whether there is 'age polythism', or a change in caste as the animals age. This is still very much under debate-- see 5, 11, and 12 for more perspective on each side.


On the left is a disperser morph; on the right a worker (or non-disperser). They are siblings: the only difference between the two is their role in the colony (caste). The grey area in the belly is adipose (fat) tissue. (Figure taken from 13)


The queen nursing young naked mole-rats (vii).


Disperser Morphs and New Colony Formation (top)

Some field work has attempted to track the movement of disperser mole-rats. It was originally suggested (13) that disperser males leave their colony to join and breed into other pre-existing colonies, thus providing gene flow and preventing too much inbreeding from occurring.

Recent work has suggested that instead, dispersers tend to form small colonies of their own. These new colonies are usually made up of dispersers from two or more nearby established colonies--according to one estimate (11), over 88% of new colonies contain animals from more than one population. Both disperser male and disperser female mole-rats form these groups, and most of them don't last very long: this same study suggests that the majority of new colonies last for less than a year (11).